The present invention relates to a system and a method for using a multipath. The present invention particularly relates to a system and a method for using a multipath in a Serial Attached SCSI (SAS) network.
Many storage networks have a multipath configuration which is a redundant configuration having duplicated paths from a host to storages. For example, in a multipath configuration in a SAS storage network, a host has two SAS initiator ports, a storage has two SAS target ports, and two redundant SAS links from the two SAS target ports of the storage are connected to the SAS initiator ports of the host, respectively. The multipath configuration allows each of the SAS initiator ports to access only a corresponding one of the SAS target ports. Even though a failure occurs on one path, this multipath configuration enables the host to access the storage through the other path. Thus, this is a necessary configuration to enhance the reliability of the storage.
However, the building-up of such a multipath configuration alone makes the host see a single storage as different storages. In addition, when a failure occurs on one path, the configuration does not allow automatic switch-over to the other path.
To cope with the problems, software for controlling the multipath (hereinafter, referred to as a “multipath driver”) is generally installed on the host. The multipath driver controls the number of storages seen from the host, detects a failure on a path, and performs switch-over between paths at the time of the failure on the path. In other words, the multipath driver makes the multipath transparent to applications of the host.
Meanwhile, techniques related to the redundant configuration in a storage network have been known conventionally (for example, see Japanese Patent Application Publication Nos. 2008-158666 and 2008-250631).
In Japanese Patent Application Publication No. 2008-158666, in a storage-device multipath system in which a host computer is connected to each storage device through multiple access paths, the host computer detects all the access paths, monitors a failure occurrence status of each of the detected access paths by issuing a check I/O to the access path at regular time intervals, and generates failure point information based on the failure occurrence statuses of the access paths, the failure point information collectively showing components each supposed to have a failure.
Japanese Patent Application Publication No. 2008-250631 describes a disk array subsystem having a network that connects multiple storage apparatuses. The disk array subsystem also includes a data input/output unit configured to input and output data from and to the network, and connecting units configured to connect the data input/output unit and each of the storage apparatuses. In the disk array subsystem, multiple physical links are established to exchange data between the data input/output unit and the connecting units and between the connecting units. Among these paths, at least one physical link is zoned into at least one or more logical links to establish multiple logical links in the one physical link. The data is transferred simultaneously in a multiplexed manner through the multiple logical links while the data is transferred through the physical link.
As described above, when a multipath configuration is built up in a storage network, a multipath driver is installed on a host in general.
However, it is necessary at present to install different multipath drivers depending on interfaces used in the storage network. Furthermore, even if a common interface is used in the entire storage network, it is necessary to install different multipath drivers on hosts in a case where the hosts have different OSs, a case where storage products or switching products of different vendors are connected to the hosts, or any similar cases. For this reason, introduction of a multipath configuration is troublesome, and further some combination of multipath drivers might cause an unexpected operation. Thus, careful combination verifications need to be performed prior to the start of the system operation.
One of conceivable methods for solving such problems is a method in which SAS expanders implement multipath control of multiple SAS paths provided between the host and storages via the SAS expanders. This method normally involves exclusive path control, but it is preferable to distribute the load on SAS traffic to the paths.
Note that the techniques in Japanese Patent Application Publication Nos. 2008-158666 and 2008-250631 are techniques related to the redundant configuration in the storage network, but are neither for solving the above problems with installation of the multipath driver on the host, nor for distributing load by using the multipath under the multipath control implemented by the SAS expanders.